New technologies, large sample collections and advances in analytic approaches have begun to contribute to the identification of genes or loci involved in complex traits. Cleft lip and/or palate (CLP) is a common birth defect and complex trait occurring in as many as 1/500 live births. Although there has been some progress in understanding the genetic components of CLP, many challenges remain. These difficulties include inheritance patterns that are not Mendelian, the involvement of environmental covariates, stochastic effects and multiple independent etiologies. Early on, twin studies proved very fruitful in confirming a strong heritable component of CLP but twins have not been used directly in gene identification. Our group has recently demonstrated that twins discordant for the autosomal dominant Van der Woude syndrome, a phenocopy of CLP, were monozygotic and had a somatic mutation found only in the affected twin. In this project we outline a novel strategy to use DNA from monozygotic (MZ) twins who are discordant for nonsyndromic CLP to search for mutations in candidate genes. We hypothesize a mutation will have arisen as a result of an early somatic mutation in one twin. The same twin pairs will also be used to explore the role that non traditional inheritance (X inactivation, imprinting or monoallelic expression) might play in CLP. Our study takes advantage of a previously collected set of MZ twins, an established sequencing project already in place and collaborators who are ideally suited to finish case collection and implement the work. Specific aims include 1) completing a collection of at least 50 MZ twins discordant for CLP, 2) confirming zygosity, 3) searching for DNA sequence variants in candidate genes, 4) measuring levels of somatic mosaicism in the affected and unaffected twin, 5) evaluating X inactivation patterns in female pairs and 6) initiating investigations of imprinting patterns and monoallelic expression. This proposal will provide specific information about the causes of non-syndromic cleft lip and palate using both genetic and epigenetic hypotheses. In addition, the study could result in proof of principle that discordant MZ twins are powerful resources for gene and/or mechanism identification in studies of complex traits in general. [unreadable] [unreadable]